It has been found in the passage of liquids through the swimming pool filter shown in applicant Harms's U.S. Pat. No. 3,720,322, issued Mar. 13, 1973, that the friction of the passage of the liquid through the filter builds up an electrostatic charge between the walls of the filter container and the electrically isolated metal plate that supports the cartridges. This is particularly the case for liquids containing ions, slight amounts of salt, and/or liquids not having a neutral pH of 7. This built-up electrostatic charge is discharged by arcing between the plate and the walls of the filter container or the handles on the plate and lid of the container, which arcing eats out or erodes the metal in the plate, handles, and/or container, even to forming holes causing the liquids to bypass the plate, and/or leak from the container, defeating the filtering operation.
This arcing, pitting and eroding of the metal in the plate, lid and side walls of the filter container around the gasket which isolates the plate that supports the cartridges, occurs whether or not pressure is employed in the container. Often times when the container is pressurized, the plate is flexed so that there is less distance or even contact between the tops of the handles and the lid or cover of the container, but this is not maintained when the pump is shut off, or a negative pressure occurs. Such a negative pressure or suction occurs when the filters are backwashed and/or mounted above the surface of the water in the swimming pool, and the back-pressure or suction due to the drop in the height of the water in the filter container above that in the pool, flexes the cartridge plates so that their handles are spaced away from the lid of the filter and thus aid in increasing an electrostatic potential which can be built up between the plate and the walls of the filter container, causing stronger arcing and resulting deeper pitting.
Accordingly, it is the purpose of this invention to prevent the build-up of such electrostatic charges and thus reduce the erosion of the metal parts of a filter due to arcing of such charges.